Increased contribution of root exudates to soil carbon input during grassland degradation

The contribution of root exudates to soil carbon (C) inputs is poorly quantified, especially for grasslands under the context of degradation that includes simultaneous shifts in species composition due to long-term overgrazing. The objectives of this study were to examine (1) how grassland degradation affects root exudation rate and (2) to what extent root exudates contribute to soil C inputs compared to litter materials and root turnover during degradation. A field experiment was conducted in a temperate grassland, with different levels of degradation (undegraded control, moderately degraded, and severely degraded), dominated by different species in Inner Mongolia. Aboveground biomass, root biomass, and root exudate rate were measured monthly during a growing season. Root exudation rates differed among dominant plant species across the degradation gradients and grassland degradation significantly increased root exudation rates at the plot level. However, the amount of root exudates released to soils in degraded sites was lower than in undegraded site due to reduced living root biomass. Compared to the undegraded control, the relative contribution of root exudates to soil C inputs was enhanced by grassland degradation as a result of decreased contributions from root turnover. These findings prove grassland degradation can alter the relative contribution of root exudates compared to other soil C inputs and highlight the necessity of further studies to examine how this shift may affect soil organic C formation and long-term storage.